Moulded plastic fused spur, switched, with flex outlet.
This would typically be used for a fixed item such as a wall mounted heater.
The appliance flex passes under the flat bar which is secured with two screws. Wires connect to the top right terminal (L), lower left (N) and the earth (centre). Note these all have brass cup washers to ensure the fine strands of the flex are all secured properly.
The rectangular space on one side is to ensure the holder can only be fitted into the plate one way.
This fuse is a modern replacement - the original would have been branded MK.